(1) Field of the Invention
The present invention relates to an electrical insulating material having an improved destructive strength to impulse voltage and containing an ethylene copolymer excellent in crosslinking properties, and relates to a power cable comprising a crosslinked insulating layer containing the ethylene copolymer, and having excellent dielectric strength and heat resistance.
(2) Description of the Prior Art
Heretofore, a variety of plastic materials have been used as electrical insulating materials for power cables and the like. Among others, olefin polymers are particularly excellent in electrical and mechanical properties, chemical stability and the like. Above all, low-density polyethylenes prepared by a high-pressure radical polymerization are inexpensive and have less dielectric loss and good workability. In addition thereto, these polyethylenes can be improved in their heat resistance when crosslinked, and in these polyethylenes, a tree phenomenon resulting from the contamination with foreign matters such as catalyst residues does not occur as often as in the case of a polyethylene prepared by ion polymerization. Since having so many advantages, the low-density polyethylenes are utilized extensively as materials for electric wires and power cables.
A problem which is now present in such an insulating material for power cables is that the wall thickness of the material must be increased in proportion to a heightened voltage when a higher transmission voltage is required in accordance with the augmentation of transmission capacity. For example, in the polyethylene materials which are now used, insulation breakdown will occur unless an insulating layer is thickened to an extreme degree in order to withstand the higher voltage.
For the solution of such a problem, various improved methods have been suggested. For example, some methods have been presented in which a graft polymerization of styrene and a polyethylene is made for the sake of improving destructive strength to impulse voltage (impulse destructive strength) particularly in a high-temperature area. One of these methods is disclosed in Japanese Patent Publication No. 18760/1979, but in this method disclosed, crosslinking of polyethylene must be carried out prior to or simultaneously with the graft polymerization of styrene and the molding method is thus limited, and there is the problem that its impulse destructive strength decreases in a low-temperature section. Japanese Patent Provisional Publication No. 80605/1982 suggests a method in which ethylene polymer grains are impregnated and polymerized with an aromatic vinyl monomer in an aqueous suspension, but this method disadvantageously comprises very complicated processes.
Other methods have been further suggested in which a polyethylene or an olefin polymer is blended with an aromatic polymer such as a polystyrene (Japanese Patent Publication No. 20717/1963, and Japanese Patent Provisional Publications Nos. 142651/1975 and 54187/1977), but a compatibility between the polyethylene or the olefin polymer and the styrene polymer is disadvantageously poor.
Another suggested method comprises adding a block copolymer of styrene and a conjugated diene to a polyethylene (Japanese Patent Provisional Publication No. 41884/1977), but this method leads to the deterioration of heat resistance and extrusion workability.
Besides, a method of impregnating an electrical insulating oil with a polyethylene has been suggested (Japanese Patent Provisional Publication No. 33938/1974), but in this method, the impregnated electrical insulating oil will be bled out during a long-term storage or by changing ambient environments, so that an effect due to the oil will be impaired inconveniently.
Further, another problem of the crosslinked polyethylene electrical cables is poor dielectric strength, which fact leads to the increase in the wall thickness of the insulating layer of the high-voltage electrical cables, that is to say, which fact causes an outer diameter of the cables to vary, with the result that the transportation and disposition of such cables will be very troublesome.
In the case that a polyethylene is employed as the insulating layer for the electrical cables, a crosslinking treatment is carried out for the improvement in heat resistance. However, in that case, the crosslinking properties of the polyethylene are not sufficient. Therefore, the improvement in high crosslinking properties, i.e., high heat resistance is being desired.